The v-myb oncogene is the transforming component of two independently- isolated chicken leukemia viruses. Activation of the c-myb proto-oncogene has been implicated in chicken, murine and human leukemias, including acute myelogenous leukemias (AML), and c-myb expression has been associated with control of cell differentiation, proliferation and the cell cycle. However, v-myb is tissue-specific, and by itself does not transform fibroblasts or other cell types, suggesting that the Myb oncoprotein cooperates with a myeloid-specific cofactor to transform cells. Interestingly, regulation of mim-1, the only known myeloid- specific cellular gene which is directly regulated by v-myb, has been shown to be absolutely dependent on a myeloid-specific transcription factor named NF-M, which is highly related to the mammalian protein known as NF-IL6 or C/EBPb. In addition, overexpression of Myb plus NF-M in heterologous cells can induce the expression of mim-1 and. other genes which are usually myeloid-specific. Thus, Myb and NF-M appear to constitute a bipartite, combinatorial code for myeloid gene activation. The parallel between mim-1 gene regulation and the tissue-specificity of transformation by v-myb is striking, and suggests that the latter process may also depend on the combinatorial action of Myb plus NF-M. We propose to examine the interactions between Myb and NF-M at several levels. Cotransfection assays will be used to determine what other myeloid-specific genes, like mim-1, can be activated by Myb plus NF-M in either chicken or mammalian cells. A truncated, dominant-negative form of NF-M, which has been shown to block the expression of the mim-1 gene, will be used to interfere with endogenous NF-M (or NF-IL6) protein in v- myb-transformed cells. This will test whether the transforming activity of Myb depends on the collaboration of active NF-M. Coexpression of v-myb and NF-M in fibroblasts will test whether the "host range" of the oncogene can be expanded when NF-M activity is supplied in trans. An analysis of a newly-identified mammalian gene related to mim-1, which is apparently regulated like its chicken counterpart, will be performed to glean new information about mammalian promoters which are regulated by the combinatoriaI signal of Myb plus NF-M. Finally, biochemical and genetic analyses will test whether Myb and NF-M bind cooperatively to DNA, interact physically with one another in a multiprotein complex, or affect each other's biological activity. These experiments will test some of the basic mechanisms of tissue- specific gene regulation, the combinatorial functions of transcription factors, and the target specificity of transforming oncogenes. The results should shed light on differentiation, growth control and the onset of leukemias in humans.